Field of the Invention
The present invention concerns a method for coating a pipe or a pipe section. In particular, the invention concerns coating steel pipes and field-joints of steel pipes. In the method, the pipe or pipe section is provided with a polymer coating for protecting the pipe or pipe section and in particular pipe joints. The polymer material is generally applied on the surface of the pipe or pipe section along at least a part of the periphery thereof.
Description of Related Art
Steel pipes coated with layers of polymers, such as polyethylene or polypropylene, have been used in oil and gas pipelines for a long time. These kinds of pipes are mechanically strong and have good corrosion resistance along the coated part of the pipe. They are generally welded together using the SAW technique (Submerged Arc Welding) to form a pipeline. To facilitate weld-joining of the pipes at the construction site, end portions of the pipe are usually left without coating at the factory. In practice, the pipes are often coated entirely and in a later stage the polymers are stripped or brushed off at the ends of the pipe. This is called cut-back area and the length is defined through the project specification, normally 150 mm on both pipe ends.
Field-welded pipe joints, i.e. girth welds formed at the construction sites, are sensitive to corrosion. For this reason, a coating layer has to be spread upon the girth weld and on the adjacent, uncoated portions of the pipes so as to completely cover the welded joint and to shield it from moisture and water in the ambience. Thus, the applied polymer layer overlaps with a factory coating of the pipes joined.
Field-joint coatings are conventionally most typically produced by injection moulding or by surface fused tape techniques. Also a number of other techniques such as epoxy-coating, PUR-coating, and coating by vulcanized tapes or shrinkable sleeves have been utilized. WO 2008/132279 discloses another method and apparatus for coating field-welded joints by applying polymer material in the form of a melt film or sheet on the surface of the pipe or pipe section from a moving die or nozzle. WO 01/32316 discloses another pipe coating apparatus for coating girth welds of pipelines, which is equipped with spray coating means.
For achieving a durable coating on the pipe, it is essential that the applied polymer layer is strong, internally of high quality and effectively bonded to the pipe. A vast number of different protective coating solutions (according to the field-joint coating standard ISO 21809-3) are considered durable in terms of sufficient protection. Powder epoxy is today used widely either as stand alone or together with a polyolefin coating. In such a case the steel pipe is heated up to desired application temperature normally using inductive heating, and the desired polymers are applied on to the surface. In one common method, the steel pipe is heated to 180-250° C. and powder epoxy is applied, fused and through cured onto the pipe.
A technique involving the combined use of epoxy powder and polyolefin has the potential to provide very durable and tight coatings. However, prior practical implementations of the technique suffer from certain disadvantages. This is because the conventionally used a steel temperature, higher than about 180° C., results in through curing of the epoxy in a short time. Thus, there needs to be an intermediate adhesive layer, e.g. polyolefin powder, application stage before the final topcoat is applied for achieving sufficient adhesion of the layers. Such approach is presented in EP 1316598, which discloses a method where an adhesive blend composition and a polyethylenic outer layer are coextruded on a through-cured epoxy layer, thus forming a three-layer final structure. However, in all these situations, the application method is unnecessarily complicated.
Thus, there is a need for improved pipe coating methods.